Heh, these things hardly ever just fit right.
There is a n adaptor plate that most time comes with the kits that takes care of the front mount. It has holes to bolt it to the engine stud holes and other holes to accept a large dia u bolt to clamp the plate to the tube.
Lotta ppl make their own adaptors from muffler clamps and a piece of plate.
I haven't had too much troubl with rear mounts that had the tube slightly too small dia. as long as the strap tightens down hard, it'll work. Sometimes I cut lead sheet flashing into narrow strips and stick them in the joint verticaly and bend over the short protruding tops so they won't fall out becore the joint is tightened..
It's real important to get the engine in place, with rearmounting bolts hand tight,and work all the slack out of the rear mount and make it fit exactly parallel on the tube, with the engine slightly too high.
Then slide ehe engine down the seat tube untilthe front adaptor plate meete the tube or minimum pedal ring or chainguard clearance is met .Now tighten the rear mount good then adapt and fasten however needed the front mount.